Dynamo electric machine construction

ABSTRACT

The rotor of a dynamo electric machine, one which is to be supported only at one end, is attached to the shaft by a diskshaped steel element of circular outline which has outer marginal portions which are cast into the conductor portions of a squirrel cage rotor, the casting being, of aluminum for light weight and high conductivity; in a preferred form, the disk-shaped steel element is dished or bowed, and the marginal portions are bentover, in contact with the magnetic elements of the rotor, or spaced therefrom by a distance sufficient to leave enough casting material adjacent the motor so that the resistivity of the shortcircuited squirrel cage windings will be hardly increased by the presence of the steel element.

'b' United States atent 1191 [111 3,777,197

Papst et a1. 4, 1973 [54] DYNAMO ELECTRIC MACHHNE 2,441,801 5/1948 Dever310/67 CQNSTRUCTXON 2,926,838 3/ 1960 Van Rijn 310/67 3,596,121 7/1971Chang 310/211 [75] inventors: Georg Papst, St. Georgen, Black Forest;Gunter w b fl Viuingen, FOREIGN PATENTS OR APPLlCATlONS Black Forest;Gerold Schneider, St. 7,006,439 11/1970 Netherlands 310/67 Georgen, allof Germany Appl. No.: 211,538

Foreign Application Priority Data Jan. 8, 1971 Germany P 2100 663.3

US. Cl. 3111/67 Int. Cl. H02k 7/00 Field of Search 310/67, 266, 211,310/182, 261, 264, 265, 62, 63, 91

References Cited UNITED STATES PATENTS 2,990,112 6/1961 Levy 310/671/1966 Papst 310/67 4/1956 Font 310/67 Primary ExaminerR. SkudyAttorney-Flynn & Frishauf [5 7 ABSTRACT The rotor of a dynamo electricmachine, one which is to be supported only at one end, is attached tothe shaft by a disk-shaped steel element of circular outline which hasouter marginal portions which are cast into the conductor portions of asquirrel cage rotor, the casting being, of aluminum for light weight andhigh conductivity; in a preferred form, the disk-shaped steel element isdished or bowed, and the marginal portions are bent-over, in contactwith the magnetic elements of the rotor, or spaced therefrom by adistance sufficient to leave enough casting material adjacent the motorso that the resistivity of the shortcircuited squirrel cage windingswill be hardly increased by the presence of the steel element.

24 Claims, 16 Drawing Figures PATENTEDBEE 41w 3.777.191

sum 1 OF 5 Fig. 1

PATENTEDBEC 41m 3.777.191

SHEET 1 0F 5 PATENTED DEB 41973 SHEET 5 [IF 5 DYNAMO ELECTRIC MACHINECONSTRUCTION The present invention relates to a dynamo electric machineand more particularly to a fractional horse power small appliance-typedynamo electric machine; and specifically to the rotor construction fora motor.

Dynamo electric machines of the fractional horse power type, when usedas motors, are widely incorporated in axial ventilators, tape recorders,phonographs and turntables, office machinery and the like. In oneconstruction which is widely used, the rotor is located outside of thestator and it has previously been proposed to make the short circuitcage of the rotor simultaneously in the form of a rotating cover 'orshell, the cover or shell-rotor combination being assembled with ashaft. The shaft itself is customarily of different material, typicallysteel, and must be interconnected with the rotor housing which isfrequently of different material, that is, material which is easy tomachine and has a low electrical resistivity. The interconnection between the shaft of steel and the housing or casting of a different metalis frequently made by casting a shaft bushing into the rotor itself.

Circumferential rotor-type rotors are usually cupshaped, the shaft ofthe rotor being secured to the bottom of the cup-shaped rotor element.The walls of the rotor element form the electromagnetically activeportion of the'motor. The bottom of the cup-shaped element usually iscast from the same material as the squirrel cage and integral with it,and is therefore, for constructional and electrical reasons, usuallymade of aluminum. In order to reduce the electrical resistance of thecage, the aluminum used is frequently highly puritied aluminum. Suchhighly purified aluminum has poor strength characteristics and thereforethe bottom of the rotor cup must be made relatively thick, and in manyconstructions it is necessary to include stiffening ribs or the like,particularly when the bottom is pierced with ventilation openings orducts. Increasing the thickness of the bottom of the rotor cup, orparticularly using ribs to improve the strength limits the size of theend turn of the windings of the stator, or requires an increase in axiallength of the entire motor which is frequently undesirable. It is adifficult foundry procedure to make thin aluminum bottoms in cup-shapedstructures. It is also difficult to reliably secure a shaft, or-a shaftbushing into the bottom of such a cup-shaped rotor structure, and toprovide for long-term secure attachment, and alignment of the shaftunder dynamic stresses, as the motor operates. Particularly if the rotoris cast of pure aluminum, alignment and matching of surfaces of theinside of the rotor with the shaft forms a critical point in the entiremotor assembly.

The development of motors is directed to decrease their axial length.This is true not only for motors in which the rotor surrounds thestator, but also for motors in which the stator is located outside ofthe rotor. In a known construction, a central support tube is providedon which bearings are mounted. The rotor is hollow on its inside andsecured to a shaft which is inserted about, or within the bearings ofthe central support tube, to turn therein, or thereover. The rotor thusrotates about the fixed shaft tube and co-acts at its outer surface withthe electromagnetic active air gap derived from the stator. The rotatingparts of the motor of this type of construction are mechanically similarto the parts of the motor in which the stator is located inside of themotor, so that the same difficulties regarding mechanical attachment ofthe end shell of the rotor will arise. The axial length of such aninterior-type rotor of a dynamo electric machine is less, but it is moreexpensive than the known construction of internal rotors in which astack of laminations are secured to the shaft of the machine itself, andin which the shaft is held in bearings fixed to the end bells of therotor.

It is an object of the present invention to provide a dynamo electricmachine which has short axial length and which is inexpensive to make,while providing reliable interconnection of the parts, so that, even inlongterm operation, they will not go out of alignment or be subject todamage.

SUBJECT MATTER OF THE PRESENT INVENTION Briefly, the dynamo electricmachine has a stator and a rotor which is rotatable with a shaft withrespect to the stator about the axis of the shaft. The rotor is securedto the shaft at least at one end portion thereof, the attachmentarrangement including a disk-shaped metal element, typically steel, ofessentially circular outline which has its outer marginal portions castinto cast portions of the rotor. The disk-shaped element is preferablydished, or bowed, and the outer marginal portions may be formed withcut-outs to further improve the intimate connection between the castrotor structure (for example of highly purified aluminum) with themechanically strong dished end metal element.

The end disk-shaped metal element may bear against the magnetic parts ofthe rotor structure, or be spaced therefrom by a distance which isselected to leave enough rotor cage material so that the resistivity ofthe end connections of the rotor is hardly increased over that it wouldhave in the absence of the end portions of the disk-shaped elements, sothat the torque is only slightly decreased. In accordance with anotherembodiment, the end marginal portions are in contact with the magneticcircuit of the rotor, the end cage connections being cast therearound.The shaft can readily be secured to the end disk, preferably beforecasting of the end plate into the remainder of the rotor structure, bylocating a shaft in a centered cut-out which is partly non-circular, andupsetting a shaft against the originally non-circular opening so thatmaterial will flow therebetween, thus forming a reliable tight andsecure connection which will not permit relative rotation of the shaftand the rotor structure when finally assembled.

The invention will be described by way of example with reference to theaccompanying drawings, wherein:

FIG. 1 is a vertical cross-sectional view through a motor structurehaving an outer rotating rotor, integrally connected with the blades ofan axial ventilator; the lower half of FIG. 1 is in a section which isrotated by an angle of with respect to the section of the upper half;

FIG. 2 is a plan view of an end blank of the rotor end unit;

FIG. 3 is a transverse cross-sectional view of the end blank of FIG. 2,after the marginal portions are bent;

FIG. 4 is a detail view, to an enlarged scale, illustrating one step inthe operation of securing a bushing to the end disk;

FIG. 5 is a tranverse cross-sectional fragmentary view of the shaftbushing after the next step of the attachment operation, part of whichis illustrated in FIG. 4;

FIG. 6 is a plan view of another embodiment of an end disk;

FIG. 7 is a transverse sectional view of the disk of FIG. 6 afterbending;

FIG. 8 is a longitudinal sectional fragmentary view of an external rotorutilizing the end disk of FIG. 7 and cast into the rotor;

FIG. 9a is a top view, and FIG. 9b a bottom view of the rotor of FIG. 8,seen along lines IXa--IXa and IXb-IXb, respectively, of FIG. 8;

FIG. I is a longitudinal cross-sectional view of a rotor to rotatewithin a stator;

FIG. 11 is a schematic cross-sectional view of an inside-rotating dynamoelectric machine, illustrating interconnection of shaft and rotor;

FIG. 12 is a cross section along line A-A of FIG. 13a;

FIG. 13a is a plan view of half of an end disk illustrating additionalair supply to the rotor;

FIG. 13b is a plan view similar to FIG. 13a illustrating a differentembodiment; and

FIG. 14 is a transverse fragmentary cross-sectional view along lines B-Bof FIG. 13b.

In the specification which follows, with reference to the drawings, theterms right and left refer to the illustration of the drawings. Likeparts in the various embodiments will be described but once and havebeen given the same reference numerals.

The external rotor motor 11 of the fan or ventilator of FIG. 1 has anouter housing 12 which is essentially a single cast or extruded unit. Aflange 14 to which a mounting plate 15 is secured is secured to one faceplate of housing 12.

Screws 16, of which only one is shown, secure a bearing tube 17 tomounting plate 15. Stator 13 of motor 11 is secured to the bearing tubeI7. The interior of the bearing tube includes a pair ofjournal bearings18, 19, of sintered or similar porous material, having a felt element 20therebetween to supply lubricant to the bearings. A pair of disks 24 aresecured in a recess 23 of plate 15, to form a first thrust bearing for ashaft 25. Shaft 25 has a groove formed at its left end into which aC-ring 26 is snapped, holding four disks 27, forming with the right faceof bearing 18 a'second thrust bearing for shaft 25 to secure the shaft25 in axial position. Thus, shaft 25 is held by the two thrust bearings24 and 18, 27, in a predetermined position, axially, the two bearings18,19 serving as radial guides for shaft 25.

A stator stack of laminations 30 is secured to the carrier tube 17,windings 31 and 31' being inserted into the stator in known manner.Electrical connections 32, 32', carried through an insulating tube 34are connected to a terminal plate 35 secured to the housing unit 12, andon which a pair of connecting contacts 33, 33' are provided tointerconnect with the conductors 32, 32'. Windings 31, 31' have'endloops 36, 37.

Shaft 25 has a bushing 41 thereon, preferably formed as a machine partand made of steel. Bushing 41 is nonrotatably secured to shaft 25. Adisk-shaped plate 42 is non-rotatably secured to bushing 41. Disk 42 isformed with eight openings 43 (FIG. 2) to permit air to passtherethrough. The outer marginal portions are formed as extending ribs44 which are cast into the left short circuit ring 45 of the outer rotorhousing 46, forming the outer portion of the motor. The outer portion 46additionally has a rotor lamination stack 47 and a right short circuitring 48. The short circuit rings 45, 48 are interconnected by rods 49,cast into grooves formed in the lamination stack 47, rods 49 and shortcircuit rings 45, 48 forming a complete squirrel cage rotor. Thematerial of the electrical conductor portion thereof, that is, the castmaterial can be aluminum, preferably purified aluminum and, in one form,highly purified aluminum of minimum electrical resistivity. The disk 42is of strong metal, preferably steel.

Fan blades are welded to the external rotor 46, of which two are shownat 52, 53.

The end disk 42 is best seen in FIGS. 2 and 3. A flat, circular disk ispunched to have the shape shown in FIG. 2. A central opening 54 isformed therein which has noncircular contours, that is, the centralopening is formed with three outwardly extending notches 55, offset fromeach other by and which will be discussed below in greater detail. Themarginal portions have cut-outs 57 punched therein so shaped that theremaining ribs 44 at their free ends have T-shaped enlargments 58. Whenthe end disk 42 is cast into the rotor, an excellent interconnection isprovided which is reliable in operation and sturdy in use, particularly,if the end marginal portions 44 are bent over, as best seen in FIG. 3,at approximately right angle to the major plane of the element 42. Asseen in FIG. 1, the diameter of the marginal portions 44 is so selectedthat they come close to the outer circumference of the shortcircuit ring45. During casting, they are so located that the distance between theends of the lamination stack 47 and that of the ends of portions 44shown as h" in FIG. 1, is as large as possible consistent with designdimensions. These two criteria, large diameter and largest possibledistance of h contribute to reduction of the resistance of the shortcircuit ring 45 which otherwise would be increased by the presence ofthe marginal portions 44. The increase in resistance due to marginalportions 44, of much higher resistivity material, typically steel, thanthe aluminum of short circuit ring 45, should result in only minordecrease of the torque delivered by the motor with respect to a motorwhich would not have the marginal portions 44 cast therein. Experimentsand actual tests have shown that the decrease'in torque due to thepresence of cast-in marginal steel portions is negligible, and is withinthe statistical field of torque values encountered in a series of suchmotors, if the distance h and the diameter of themarginal portions 44are selected to be as large as possible.

Shaft 25 and bushing 41 should be securely interconnected, and have longoperating life. Various interconnections can be used. In one example,bushing 41 can be welded to the disk 42, particularly by pulse welding,or by brazing, for example by induction brazing. A particularly simpleinterconnection, suitable for an end disk as seen in FIGS. 2 and 3, andformed with notches 55, is illustrated in connection with FIGS. 4 and 5.

Disk 42, with a bushing 41 of raw blank form as seen in FIG. 4 isinserted into ajig 60. A hold-down tool 61 is placed against the disk42, to securely hold disk 42 and bushing 41 together. Compressive forceis applied in the direction of the arrows in FIG. 4 against acylindrical punch 62. Punch 62 has a work face 62 which is slightlyconical, and punch 62 thus will deform the outer circumference ofbushing 41 to result in the final shape shown in FIG. 5, forming'aholding ring 63 and simultaneously flowing the material of bushing 41into the notches 55 (FIG. 2) in order to provide a stable,

sturdy non-rotating connection between disk &2 and bushing 41. The loweredge 64 of bushing 41 bears against the disk 42 and is countered at theother side by the ring 63, thus forming a connection which is notsubject to change in orientation even under substantial dynamicstressing. If necessary, the bushing can be machined along thechain-dotted line 65, by cutting off the excess portion, to result inbushing 41 as seen in FIG. I, of minimum axial length. Thisinterconnection is suitable for any one of the embodiments of the rotorshaft described herein.

FIG. 8 illustrates a rotor 76 for a circumferentialrotor type dynamoelectric machine, which could be used instead of rotor 46 in the motorof FIG. I. Rotor 70 includes a lamination stack 71, with a cast shortcircuit squirrel cage, for example of pure aluminum. An upper shortcircuit ring 72 and a lower short circuit ring 73 are shown,interconnected by connecting bars which are placed in grooves (notshown) of lamination stack 71. FIG. 5 illustrates a casting apparatus tocast the end plate into the stator assembly. Circumferential edges 79,79' of a casting apparatus 69, 69 are illustrated in the position readyfor casting. In this position, the edges 79, 79 are pressed against theend plate 74 with sufficient force that they will form a shallow groovein the end disk 7 for example to the extent of 1/ l0 mm. This slightgroove, coupled with the pressure of application provides a secure andtight connection against rotor short circuit material, even ifintroduced by pressure casting. The surfaces of the disk 74 will thenhave a pair of ring-like depressed zones, of shallow depth, whichadditionally contribute to stiffening of the end disk.

The end disk 74 is secured to the upper short circuit cage 72, bycasting its marginal portion into the ring 72 forming the short circuitcage. Again, the distance between the ends of the marginal portion ofdisk 74 and lamination stack 71 is h, this space being completely filledby cast material of the end ring of the cage, in order to decrease theinternal resistance of the end ring as much as possible. FIG. 6illustrates the punched disk before profile formation, and flat, as itwould come from a punch. The disk of FIG. 6 is then shaped to have theform of FIG. 7, with the marginal portions 75-bent over. Holes 76 arepunched into the marginal portion to provide for good and intimateinterconnection of the end disk with the short circuit ring 72. Duringbending of the marginal portions 75, the disk 74 is slightly bowed, asillustrated in FIG. 7 by angle a. This additionally contributes tostiffness of the disk 74. A bushing 78 is inserted into a centralopening 77 for interconnection with a shaft, not shown in FIG. 8. Thisbushing 77 may again be inserted, by means of circumferential notches(not shown), similar to that described in connection with FIGS. 4 and 5,or welded or brazed, as desired.

The pressure casting dies 69, 69', with their circumferential pressureedges 79, 79' hold the disk 74 with such force that a sufficientdeformation occurs at the surface of the disk. This effectively preventsundesired thin flash projections, or undesired material at the surfaceof disk 74. Pin 82 of the tool centers the disk 74 during casting. Theslight deformation of the disk, particularly in combination withprevious bowing thereof, effectively stabilizes the shape andorientation of the disk with respect to the rotor after final casting.

FIG. 9a is a view of the rotor 76 of FIG. 8 from above. FIG. 9b is aview of the same rotor from below. Holes (FIG. 9a) and holes 87 (FIG.9b) are provided to permit balancing of the rotor.

FIG. It) illustrates a rotor 85 which is intended to operate within astator. Rotor 85 again is a squirrel cage rotor, with an upper shortcircuit ring 86, a lower short circuit ring 67 and connecting bars (notshown) cast into a rotor lamination stack 86. The upper ring 86 has adisk-shaped member 69 cast therein which, again, is a punched disk. Theinner opening 92 of disk 89 has shaft 911 press-fitted therein. Thepunched disk 89 should be punched with an accurate highly reliable tool,which has a very small cutting gap, so that the cut surface will be trueto size over the entire thickness of the punched part, and does not showburrs or broken away zones of excessive tolerance. If disk 89 isaccurately punched, shaft M can be press-fitted by means of aninterference fit directly, or can be interconnected by means ofupsetting, without an additional bushing.

FIG. II illustrates, schematically, a rotor construction particularlysuitable for a rotor running within a stator, and enabling use of aparticularly long bearing tube. Rotor III thus rotates within stator11111. A stationary bearing tube M2 is located in the interior of rotorIII), constructed similarly to bearing tube I7 in accordance with FIG.I. Bearing tube I I2 has a pair of self-lubricating bearings 113associated therewith, and shaft 1M rotates within the bearings. Theupper end of shaft 1114 has a bushing I15 secured thereto. A hatshaped,upset sheet metal disk 1116 is secured to bushing 115. The attachment isat a center, convex portion. The outer marginal portions of the sheetmetal element H6 bears directly against the lamination stack 117 ofrotor Ill), and is secured to the rotor 1110 by casting into the uppershort circuit ring I I8. Thus, the end disk I16, in this construction,forms one lamination of the lamination stack of the rotor III).

The disk-shaped element 74' of FIGS. 12-14 is similar to that of element74 of FIGS. 8, 9a, 9b. In addition to the openings formed therein,projections and depressions I30 are formed in the disk-shaped element74, in order to increase the amount of cooling air supplied to theinterior of the motor, when the disk rotates during operation of themotor. The projections and depressions I31, 132, as seen in FIGS. I2 and13a, are bent towards both sides of the element 74, and are arranged intwo circumferential rings I311, 132 and 131i, I32, oriented reverselywith respect to each other so that air will be supplied regardless ofdirection of rotation of the rotor. A similar arrangement is shown inFIG. 13b, where disk 74 has flags 1133, I34 punched out, the flags I33,1134 being of greater radial width and so arranged that they will supplyair to the interior of the rotor regardless of direction of rotation ofthe motor.

Rotors, and particularly short circuit rotors of fractional horse powermotors can readily be interconnected with associated shafts bypracticing the invention as disclosed. The shafts of these rotors willbe securely and reliably connected and will be in alignment with therotors, even after long periods of operation. The axial length of themotor can be reduced. The casting operation itself is simple andpresents less difficulties and does not require any special tools orfixtures.

Various changes and modifications may be made within the inventiveconcept.

We claim:

1. External rotor dynamo electric machine having a Support,

a central stator fixed in the support,

a shaft,

an external rotor rotatable with the shaft with respect to the statorabout the axis of the shaft, said rotor surrounding the stator andhaving a magnetic circuit, axially extending conductor portions andshort circuit rings located at the ends of the axially extendingconductors, the short circuit rings comprising good electricallyconductive metal integrally connected to the axially extending portionsand forming therewith an external squirrel cage rotor;

and means securing the external squirrel cage rotor and the shaft forjoint rotation together at one end portion of the rotor, said meanscomprising a disk-shaped sheet metal element of structurally strongmaterial and of essentially circular outline extending diametrically ofthe motor, said element being centrally secured to the shaft and havingouter marginal portions formed with projecting portions the projectingportions extending in axial direction and being cast into and integrallysecured within one of the electrically conductive short circuit rings ofthe rotor.

2. Machine according to claim 11, wherein the diskshaped element isconcavely bowed.

3. Machine according to claim l, wherein the diskshaped element is apunched sheet metal disk.

4. Machine according to claim 1, wherein the outer marginal portions ofthe sheet metal element are formed with radial cut-outs, the remainingmetal of the outer marginal portions forming projecting ribs, said ribsbeing cast into the adjacent short circuit ring of the rotor.

5. Machine according to claim 4, wherein the ribs are, in plan view,essentially T-shaped, and are formed with circumferentially extendingbulges.

6. Machine according to claim 1, wherein the short circuit ringscomprise high conductivity aluminum and the projecting portions are ribscast into the adjacent short circuit ring of the rotor. I. I

7. Machine according to claim ll, wherein the axially extending marginalportions form projecting ribs which are speced from the magnetic circuitof the rotor by a distance (h) determined by the current flow in theshort circuit rings, said distance (h) being selected to introduceadditional resistance into the short circuit rings which is small withrespect to the resistance of the rings without the marginal portions tominimize power loss of the machine.

8. Machine according to claim ll, wherein the marginal portions of thedisk-shaped elements are mechanically connected with the magneticcircuit of the rotor and form a portion of the rotor stack and aresecured thereto by the conductor material of the rotor.

9. Machine according to claim 1, wherein the diskshaped element hasaxially extending end portions to form a cup-shaped element.

10. Machine according to claim 1, wherein the diskshaped element has acentral cylindrical projection, the central projection being connectedwith the shaft.

11. Machine according to claim ll, wherein the diskshaped element has acentral opening which has portions which are non-circular;

a bushing pressed into the central opening and being deformed such thatthe material of the bushing flows in the non-circular portions to form anonrotatable connection between the bushing and the sheet metal element.

l2. Machine according to claim ll, wherein the sheet metal element has acentral opening;

a bushing secured in the central opening, the bushing being welded tothe sheet metal element.

1135. Machine according to claim l, wherein the sheet metal element isformed with a central opening; a bushing inserted into the centralopening, the bushing being brazed to the central opening.

ill. Machine according to claim 11, wherein the sheet metal element is aprecision punched part having a central opening punched therein, thecentral opening being held to accurate tolerance throughout thethickness of the sheet metal element;

the shaft being press-fitted into said central opening.

15. Machine according to claim ll, wherein the sheet metal element isformed, along its face, with projecting fins punched from the plane ofthe sheet metal element to provide openings through the sheet metalelement and air scoops for ventilation of the interior of the rotor.

16. Machine according to claim 16, wherein the punched fins are locatedon concentric circles of the sheet metal element, selected finsprojecting in opposite directions to provide air scoops for the interiorof the motor regardless of direction of rotation of the rotor.

17. Machine according to claim ll, wherein the sheet metal element isdeformed by circumferential shallow rings extending concentrically withthe shaft into the planeof the sheet metal element.

118. Motor according to claim 1, wherein the diskshaped sheet metalelement is of steel.

19. Fractional horse power motor having a shaft, a stator surroundingthe shaft windings surrounding the stator, and an external squirrel cagerotor surrounding the stator and having an electromagnetic portioncomprising axial conductor bars and integrally connected short circuitend rings comprising a material of good electrical conductivity but oflow mechanical strength, and stacks of magnetic laminations being castbetween the short circuit end rings;

and a sheet metal disk of a material of high mechanical strength and anelectrical conductivity which is low with respect to that of theconductor end rings and bars, formed with a central opening, the shaftbeing secured to the central opening, transversely of the disk with aninterference fit to locate the disk adjacent an end portion of thestator;

the sheet metal disk having axially extending projecting marginalportions which are cast into and integrally secured into the adjacentshort circuit end ring to secure the electromagnetic portion of therotor directly to the shaft and form a unitary, integral rotor-shaftassembly which can rotate about the stator.

20. Motor according to claim 19, wherein the end rings and conductorbars are aluminum.

21. Motor according to claim 19, wherein the sheet metal disk is steel.7

22. Motor according to claim 19, wherein the marginal portions of thesheet metal disk are perforated MB 24. Machine according to claim 9,wherein the outer marginal portions of the disk-shaped sheet metalelement comprise radially extending end portions joined to the axiallyextending portions, the radially extending portions being connected withthe magnetic circuit of

1. External rotor dynamo electric machine having a support, a centralstator fixed in the support, a shaft, an external rotor rotatable withthe shaft with respect to the stator about the axis of the shaft, saidrotor surrounding the stator and having a magnetic circuit, axiallyextending conductor portions and short circuit rings located at the endsof the axially extending conductors, the short circuit rings comprisinggood electrically conductive metal integrally connected to the axiallyextending portions and forming therewith an external squirrel cagerotor; and means securing the external squirrel cage rotor and the shaftfor joint rotation together at one end portion of the rotor, said meanscomprising a disk-shaped sheet metal element of structurally strongmaterial and of essentially circular outline extending diametrically ofthe motor, said element being centrally secured to the shaft and havingouter marginal portions formed with projecting portions the projectingportions extending in axial direction and being cast into and integrallysecured within one of the electrically conductive short circuit rings ofthe rotor.
 2. Machine according to claim 1, wherein the disK-shapedelement is concavely bowed.
 3. Machine according to claim 1, wherein thedisk-shaped element is a punched sheet metal disk.
 4. Machine accordingto claim 1, wherein the outer marginal portions of the sheet metalelement are formed with radial cut-outs, the remaining metal of theouter marginal portions forming projecting ribs, said ribs being castinto the adjacent short circuit ring of the rotor.
 5. Machine accordingto claim 4, wherein the ribs are, in plan view, essentially T-shaped,and are formed with circumferentially extending bulges.
 6. Machineaccording to claim 1, wherein the short circuit rings comprise highconductivity aluminum and the projecting portions are ribs cast into theadjacent short circuit ring of the rotor.
 7. Machine according to claim1, wherein the axially extending marginal portions form projecting ribswhich are speced from the magnetic circuit of the rotor by a distance(h) determined by the current flow in the short circuit rings, saiddistance (h) being selected to introduce additional resistance into theshort circuit rings which is small with respect to the resistance of therings without the marginal portions to minimize power loss of themachine.
 8. Machine according to claim 1, wherein the marginal portionsof the disk-shaped elements are mechanically connected with the magneticcircuit of the rotor and form a portion of the rotor stack and aresecured thereto by the conductor material of the rotor.
 9. Machineaccording to claim 1, wherein the disk-shaped element has axiallyextending end portions to form a cup-shaped element.
 10. Machineaccording to claim 1, wherein the disk-shaped element has a centralcylindrical projection, the central projection being connected with theshaft.
 11. Machine according to claim 1, wherein the disk-shaped elementhas a central opening which has portions which are non-circular; abushing pressed into the central opening and being deformed such thatthe material of the bushing flows in the non-circular portions to form anon-rotatable connection between the bushing and the sheet metalelement.
 12. Machine according to claim 1, wherein the sheet metalelement has a central opening; a bushing secured in the central opening,the bushing being welded to the sheet metal element.
 13. Machineaccording to claim 1, wherein the sheet metal element is formed with acentral opening; a bushing inserted into the central opening, thebushing being brazed to the central opening.
 14. Machine according toclaim 1, wherein the sheet metal element is a precision punched parthaving a central opening punched therein, the central opening being heldto accurate tolerance throughout the thickness of the sheet metalelement; the shaft being press-fitted into said central opening. 15.Machine according to claim 1, wherein the sheet metal element is formed,along its face, with projecting fins punched from the plane of the sheetmetal element to provide openings through the sheet metal element andair scoops for ventilation of the interior of the rotor.
 16. Machineaccording to claim 16, wherein the punched fins are located onconcentric circles of the sheet metal element, selected fins projectingin opposite directions to provide air scoops for the interior of themotor regardless of direction of rotation of the rotor.
 17. Machineaccording to claim 1, wherein the sheet metal element is deformed bycircumferential shallow rings extending concentrically with the shaftinto the plane of the sheet metal element.
 18. Motor according to claim1, wherein the disk-shaped sheet metal element is of steel. 19.Fractional horse power motor having a shaft, a stator surrounding theshaft windings surrounding the stator, and an external squirrel cagerotor surrounding the stator and having an electromagnetic portioncomprising axial conductor bars and integrally connected short circuitend rings comprising a material of good electrical conductivity but ofLow mechanical strength, and stacks of magnetic laminations being castbetween the short circuit end rings; and a sheet metal disk of amaterial of high mechanical strength and an electrical conductivitywhich is low with respect to that of the conductor end rings and bars,formed with a central opening, the shaft being secured to the centralopening, transversely of the disk with an interference fit to locate thedisk adjacent an end portion of the stator; the sheet metal disk havingaxially extending projecting marginal portions which are cast into andintegrally secured into the adjacent short circuit end ring to securethe electromagnetic portion of the rotor directly to the shaft and forma unitary, integral rotor-shaft assembly which can rotate about thestator.
 20. Motor according to claim 19, wherein the end rings andconductor bars are aluminum.
 21. Motor according to claim 19, whereinthe sheet metal disk is steel.
 22. Motor according to claim 19, whereinthe marginal portions of the sheet metal disk are perforated and theregions of marginal portions beyond the perforations are cast into therotor-shaft assembly.
 23. Motor according to claim 19, wherein thecentral opening of the shaft, at least in part, is noncircular; abushing press-fitted into the opening and having bushing material flowedinto the non-circular portions of the opening; the bushing being securedto said shaft.
 24. Machine according to claim 9, wherein the outermarginal portions of the disk-shaped sheet metal element compriseradially extending end portions joined to the axially extendingportions, the radially extending portions being connected with themagnetic circuit of the rotor.